Extremal subsets and atom-positivity

Abstract

Demazure crystals give a combinatorial framework in which to study Demazure modules. They are extremal, in that they satisfy Kashiwara's string property, and they are Demazure atom-positive, in that they decompose naturally into subsets called crystal Demazure atoms. The properties of extremality and atom-positivity are further linked by a conjecture of Polo, which concerns the structure of the tensor product of two Demazure modules. We study these two properties in isolation - first providing a recursive formula for generating crystal Demazure atoms which generalizes a result of Lascoux-Sch\"{u}tzenberger, and then examining the structure of extremal subsets and their characters - before commenting on the connection (or lack thereof) between them.